With the increasing down-scaling of integrated circuits and increasingly demanding requirements to the speed of integrated circuits, transistors need to have higher drive currents with increasingly smaller dimensions. Fin Field-Effect Transistors (FinFETs) were thus developed. A typical FinFET includes a semiconductor fin above a substrate, which fin is used to form the channel region of the FinFET. The channel region includes sidewall portions and sometimes a top surface portion of the semiconductor fin. When the channel region includes the sidewalls portions but not the top surface portion, the respective FinFET is referred to as a dual-gate FinFET. When the channel regions include the sidewalls portions and the top surface portion, the respective FinFET is referred to as a tri-gate FinFET.
To improve the channel control and reduce leakage currents in the FinFETs, it is desirable that the widths of the semiconductor fins of the FinFETs are reduced. In conventional methods, the fins were oxidized to reduce the widths. The oxidation was performed after the trenches for Shallow Trench Isolation (STI) regions have been formed by etching a semiconductor substrate. The portions of the semiconductor substrate between the trenches thus form semiconductor strips, which are thinned through oxidation. After the oxidation, the resulting oxide is removed, and hence the semiconductor strips become thinner. The conventional methods, however, suffer from yield and performance issues. When the widths of the semiconductor strips are reduced, the semiconductor strips may be distorted, resulting in the degradation in the performance of the FinFETs or the loss of yield.